1. Field of the Invention
The present invention relates to a fuel supply control method for internal combustion engines and, more particularly, the present invention relates to a fuel supply control method of controlling fuel supply to a multicylinder internal combustion engine by means of two fuel injection valves provided in an intake pipe upstream and downstream of a throttle valve, respectively.
2. Description of the Prior Art
Japanese Provisional Patent Publication (Kokai) No. 47-35422 discloses a fuel supply control system which controls a single fuel injection valve to supply fuel to the cylinders of a multicylinder internal combustion engine. This known fuel supply control system controls a main fuel injection valve, namely, an ordinary fuel injection valve having a large capacity, disposed upstream of a throttle valve with respect to the direction of flow of intake air during middle load and high load operation of the engine and controls an auxiliary fuel injection valve disposed downstream of the throttle valve with respect to the direction of flow of intake air during low load operation of the engine. A fuel injection valve having excellent fuel atomizing characteristics is employed as the auxiliary fuel injection valve to secure uniform distribution of fuel to all the cylinders during low load operation of the engine.
In such a fuel supply control process, the working fuel injection valve is changed over between the auxiliary fuel injection valve (hereinafter referred to as "downstream valve") and the main fuel injection valve (hereinafter referred to as "upstream valve") when the operating mode of the engine changes from a middle load mode or a high load mode to a low load mode or vice versa. However, when the working injection valve is changed over from the downstream valve to the upstream valve, it occurs that an appropriate amount of fuel which actually contributes to combustion is not surely supplied to the engine at a moment immediately after the changeover of the working injection valve due to the positional difference between the downstream valve and the upstream valve. That is, fuel injected by the downstream valve is supplied directly to the cylinders of the engine only through a portion of the intake pipe extending downstream of the downstream valve, whereas part of the fuel injected by the upstream valve stays temporarily on inner surfaces of the throttle body and surfaces of the throttle valve before flowing into the cylinders of the engine. Consequently, when the operation of the downstream valve is stopped and the operation of the upstream valve is started to supply the same amount of fuel as the downstream valve, an insufficient amount of fuel is supplied temporarily to the engine, adversely affecting the driveabilitry of the engine because fuel injected by the upstream valve at the start of the same stays temporarily on inner surfaces of the throttle body and surfaces of the throttle valve.
Furthermore, the fuel supply amount of fuel supplied to the engine from the upstream valve and the downstream valve is proportional to the fuel injection period for which the injection valve is open. Accordingly, during high load operation of the engine requiring a large fuel supply quantity the fuel injection period has to be increased. However, since the period of suction stroke of the engine diminishes as the engine speed increases, the fuel injection period of the fuel injection valve can be longer than the period of suction stroke of the engine. In such an event, part of the fuel injected by the fuel injection valve provided upstream of the intkae manifold in sychronism with and for a period beyond the period of suction stroke of one cylinder is sucked in the other cylinders. Accordingly, the fuel injection period must not be longer than the period of suction stroke. Under such circumstances, when a large amount of fuel is injected in a short fuel injection period by a fuel injection valve having a large capacity (large nozzle bore), the accuracy of fuel injection control and the atomization of the injected fuel are deteriorated in injecting a small amount of fuel for low load operation of the engine. On the other hand, when a fuel injection valve having a small nozzle bore is used to enable satisfactory atomization of fuel during low load operation of the engine, the maximum fuel flow rate that can acurately be metered is limited to a low level.